Administration of intravenous fluids to a patient is well known in the art. Typically, a solution such as saline, glucose or electrolyte in a glass or flexible container is fed to a patient's venous access site via a length of flexible plastic tubing such as polyvinyl chloride (PVC) tubing. The rate of flow of the fluid is controlled by a roller clamp which is adjusted to restrict the flow lumen of the tubing until the desired flow rate is obtained.
Flow from the container to the patient may also be regulated by means other than a roller clamp. It is becoming more and more common to use an electronically controlled pump. One type of pump that is used for intravenous fluid administration is a peristaltic-type pump.
Use of peristaltic pumping action is particularly well suited for the medical field. This is because peristaltic pumping action can be applied externally of the tubing carrying the intravenous fluid. This maintains the sterile condition of the intravenous fluid within the tubing while imparting fluid propulsion on the fluid. The peristaltic pumping action can also be applied at any point on the tubing.
A peristaltic pump is also particularly useful as the pump can be applied at any point on tubing to provide fluid propulsion. In a common type of peristaltic pump used in the medical field, a driving motor is connected to an array of cams angularly spaced from each other. The cams in turn drive cam followers connected to corresponding pressure fingers. These elements cooperate to impart a linear wave motion on the pressure fingers. A pressure plate is secured juxtaposed and spaced from the pressure fingers. The pressure plate holds the tubing against the reciprocating pressure fingers to impart the wave motion on the tubing to propel the fluid.
A problem associated with peristaltic pumps of this type is that over long periods of infusion such as 24 hours or longer, the diameter of the tubing can vary. If the diameter of the tube changes, the flow rate will also change. This variance can result from a change in the temperature of the fluid being infused, a change in the air temperature in the room, a variance in the downstream pressure from the patient resistance, a variance in the upstream pressure from the source of fluid, and a breakdown in the resiliency in the tubing subject to the pumping action.
Particularly important in accounting for changes in the flow rate of the fluid is the breakdown in the tubing resiliency. This results in a flattening of the tubing subject to the pumping action. This flattening results in a drop in the amount of fluid subject to the pumping action which in turn results in a drop in the fluid delivery rate over time. This can be referred to as hysteresis.
Hysteresis can be solved manually by changing the orientation of the tubing, thereby exposing a different length of tubing to the pumping action. This solution is not satisfactory for several reasons. Initially, moving the tubing results in an interruption of the fluid flow. Additionally, a nurse or other hospital worker must take the time to move the tubing.
Another solution is to speed up the rate of the motor during infusion according to a predetermined schedule. While this will result in an improved delivery accuracy, it is also not entirely satisfactory for several reasons. Initially, small variances in the tubing width can result in a different infusion rate from one segment of tubing to another. In addition, each segment of tubing exhibits a different rate of breakdown in resiliency. Further, if the tubing is replaced or the orientation of the pressure fingers is changed on the same tubing, the predetermined schedule of rate increase may actually result in a decrease in accuracy. Finally, this system fails to account for other causes of diameter variance.
What is thus needed is a device which improves the accuracy of the fluid flow of a peristaltic pump by taking into account the various factors which change infusion rates. The present invention provides such a device.